在iOS上使用后台任务进行UI更新

我有一个同步过程有时（甚至很less）需要20秒。 我曾经在后台调用select器。 有一个进度指示器，停止同步完成通知的animation，并且下面的代码立即改变：

[self performSelectorInBackground:@selector(updateFunction) withObject:nil]; 

但为了让进度继续下去，我决定使用后台任务：

  UIApplication *application = [UIApplication sharedApplication]; //Get the shared application instance __block UIBackgroundTaskIdentifier background_task; //Create a task object background_task = [application beginBackgroundTaskWithExpirationHandler: ^ { [application endBackgroundTask: background_task]; //Tell the system that we are done with the tasks background_task = UIBackgroundTaskInvalid; //Set the task to be invalid //System will be shutting down the app at any point in time now }]; //Background tasks require you to use asyncrous tasks dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ //Perform your tasks that your application requires NSLog(@"\n\nRunning in the background!\n\n"); [self updateFunction]; [application endBackgroundTask: background_task]; //End the task so the system knows that you are done with what you need to perform background_task = UIBackgroundTaskInvalid; //Invalidate the background_task NSLog(@"Finished in the background!"); }); 

但是现在，当任务完成时，通知被发送（并执行），但表中的单元格不会更新，直到（有时）相当长的时间。 所以在我看来，“需求显示”不是从后台任务等等来的时候触发的，而是只是在一个常规的刷新间隔内进行更新。

有没有办法让表格单元更新使用后台任务，或者我错过了什么？

编辑：通知监听器是：

 [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(refreshBttn) name:@"finishedUpdate" object:nil]; 

代码是：

 - (void) refreshBttn { NSLog(@"Refresh Buttun action"); iDomsAppDelegate *delegate = (iDomsAppDelegate *)[[UIApplication sharedApplication] delegate]; if([delegate updating]){ [_updateBttn setHidden:TRUE]; [_activityIndicator startAnimating]; } else { [_updateBttn setHidden:FALSE]; [_activityIndicator stopAnimating]; } [self setNeedsDisplay]; [self setNeedsLayout]; } 

Answer 1

你需要分离背景和前景线程之间的逻辑。 您要发送的通知应该从主线程发送，这样您的UI对象可以安全地进行更新。 所以，把updateFunction分成两块。 也许是这样的：

 NSLog(@"\n\nRunning in the background!\n\n"); [self updateFunctionBackground]; dispatch_async(dispatch_get_main_queue(), ^{ // send your notification here instead of in updateFunction [[NSNotificationCenter defaultCenter] post...]; }); 

Answer 2

为了让用户界面更新，你必须从后台线程调用主线程的函数，如下所示：

 [self performSelectorOnMainThread:@selector(updateFunction) withObject:nil waitUntilDone:NO]; 

如果您需要传递任何variables，则可以继续将其添加到-withObject

Answer 3

您可以确保通过您想要的任何队列收到通知…

 [[NSNotificationCenter defaultCenter] addObserverForName:SomeNotification object:sender queue:operationQueue usingBlock:^(NSNotification *){ // Your notification will be processed in this block, and it will be // received on the notification queue you specify, regardless of what // thread the sender was running when the notification was posted. }]; 

如果你想确保在主线程上收到它，你可以使用

 [NSOperationQueue mainQueue]